In a typical gas-phase, fluidized bed olefin polymerization process, fine polymer particles are carried overhead from the reactor and are removed from the recycle gas stream using cyclone or centrifugal separators. The collected fines are drawn from the bottom of the separator using an ejector and from there are returned to the reactor. Such a system is described in U.S. Pat. No. 4,882,400; the entire content of which is hereby incorporated by reference.
Ejectors currently in use and discussed specifically in the '400 patent have a horizontal design. A horizontally oriented fines ejector is shown in FIG. 1. In a typical horizontal fines ejector 10, a gas stream 11 enters the ejector 10 horizontally as shown in FIG. 1. The gas stream 11 provides a motive force to draw fines and gas 12 from the separator (not shown), incorporating them in the gas stream 11, and conveying them 13 to the reactor (not shown) into which they are subsequently re-injected.
In a continuous gas-phase, fluidized bed polyolefin polymerization process, it is common that gas loop piping and equipment would foul to the extent that a shutdown is periodically required for cleaning. Fouling is especially severe in the horizontal fines ejector 10 as the polymer powder-laden gas stream 12 from the fines separators (not shown) is forced to change direction, typically 90°, within the ejector 10. The powder and sometimes even sheets of polymer tend to build-up in area 14 and reduce the efficiency or even plug the ejector 10.
Fouling is so severe in the conventional-design ejector that cleaning to remove polymer build-up is required on average every two months, resulting in frequent plant shutdowns, excessive cleaning expenses, and unacceptable lost production. In extreme cases, heavy fouling of ejector internals will significantly impede ejector performance and has resulted in flow blockages and unplanned reactor shutdowns.
One of the aims of this invention, therefore, is to solve the problems associated with horizontal fines ejectors and extend the time between cleanings of the ejectors.